The second requirement calls for a fresh air inlet or fresh air pipe. This is no less important than the extension of the soil pipes through the roof. In order to effect a constant movement and change of air in the pipes, two openings are required, an outlet and an inlet. The extension of the soil pipe through the roof provides only an escape for the foul air generated in the soil pipes and waste pipes through the decomposition of foul organic matter, clinging to the interior of pipes and lodging in traps under water closets and fixtures. But in order to oxidize and thus render harmless this matter undergoing putrefaction within the pipes, a constant introduction of fresh air from the outside atmosphere is necessary. As the soil pipe is warmer in winter time (being in the constantly heated house) than the fresh air pipe, located outside of it, an almost continuous upward current in the soil pipe results. In summer time this current is only seldom reversed; for as a general rule, the top of soil pipe is heated by the sun more than the fresh air pipe near the ground.

There is a second and almost equally important reason for providing a fresh air inlet, wherever the third requirement, the trapping of the drain, has been complied with. If a water closet is used or a pail emptied into a slop sink, the water discharged into the soil pipe acts like a piston; although it is not likely to fill a 4-inch pipe, it certainly carries the air on its course downward with it by friction. Thus the descending water drives air before it and out through the fresh air pipe; if this had not been provided, it would very likely force the nearest traps under fixtures, and send a puff of sewer gas into the living rooms. This reversed action of the fresh air inlet does not occur sufficiently often to warrant the apprehension of any danger in the location of the inlet. Of course, it should not be too near under windows of living rooms or dormitories, nor should it be placed too near the front steps of a city house. A

G6 little judgment should be exercised in locating the fresh air inlet. In cities, having between the house and the street a wide parking, it is best to build in this a small manhole, at the bottom of which the trap and opening for fresh air are located. The top of manhole should then be closed with a cover, having numerous openings so as to permit the outer air to enter the drain freely, and also to prevent as much as possible obstructions by snow or ice in winter time. For this reason it cannot be recommended to open the fresh air pipe into a gully in the sidewalk, or in the floor of an area. Equally objectionable is the location of the fresh air pipe in a coal slide. It seems best to carry the fresh air pipe some distance away from the house, and this is always practicable in the case of country houses, where the fresh air pipe should preferably be hidden from view by shrubbery.

If the main trap is located inside the foundation walls, the fresh air pipe should enter the drain just above the trap by a T or Y branch. Only in rare cases does it become necessary to carry the fresh air pipe vertically upward through the roof. This plan would neither be very efficient, as the difference in temperature of inlet and outlet pipe would be small, nor very economical.

As regards size of the fresh air pipe, I would say that nothing short of the diameter of the iron drain would answer; as this is generally 4 inches in diameter, a 4-inch opening for fresh air pipe is required. This opening should be protected against obstructions by a wire basket similar to that used for the upper part of soil or waste pipes.